H / F / Bhardness / firm / black

What are the meanings of H, F, and B respectively?

Symbol: H, F, B / Base data: Hardness.
Symbol expressing the hardness of the lead of a black lead pencil

Expressing hardness and thickness

Let’s reexamine a pencil lead. H means “hard” and B is “black”. The higher the H number the harder the lead, and the higher the B number, the thicker and softer the lead. HB is in the middle. F means “firm” and the thickness and softness lies between H and HB.